1. Field of the Invention
The present invention relates to a golf ball having a polyurethane cover.
2. Description of the Background Art
There has been known a golf ball cover formed by ionomer resin or polyurethane as a base resin thereof. While a golf ball cover using ionomer resin is widely used by reason of its advantages in resilience, durability and processability, it is pointed out that its high rigidity and hardness causes problems, such as poor shot feeling, and inferior controllability due to difficulties in obtaining a sufficient spin performance. As one measure against this problem, Japanese Patent No. 2709950 discloses a technique of forming a golf ball cover using a blend of hard ionomer resin and soft ionomer resin to provide enhanced shot feeling and spin performance. The soft ionomer resin blended therein, however, inevitably causes deterioration in resilience and/or scratch or abrasion resistance. In view of the capability of providing enhanced shot feeling and/or spin performance as compared to ionomer resin, polyurethane is also used as a base resin of a golf ball cover.
The polyurethane for use as a base resin of a golf ball cover includes thermosetting polyurethane or thermoplastic polyurethane. The use of the thermosetting polyurethane allows a crosslinked structure to be formed so as to provide enhanced abrasion resistance in the cover. On the other hand, it involves a problem of deterioration in productivity due to complicated production processes, such as the need for adjusting its viscosity in a process of coating a golf ball core with the cover. In case of using the thermoplastic polyurethane, while a coating process can be performed by injecting a cover composition directly onto a core, so as to achieve excellent productivity as compared to the case of using the thermosetting polyurethane, the thermoplastic polyurethane having a straight-chain structure causes difficulties in obtaining sufficient abrasion resistance during ball hitting.
As measures against such a problem, Japanese Patent Laid-Open Publication No. 11-178949 proposes a golf ball with a cover formed by a product of the reaction between a thermoplastic polyurethane elastomer and an isocyanate compound. This technique involves difficulties in stably molding the cover, because the thermoplastic polyurethane elastomer is melted only at a high temperature, and the reaction with the isocyanate to be caused by mixing them under a high temperature is undesirably accelerated. Japanese Patent Laid-Open Publication No. 2002-360741 also proposes a golf ball with a cover formed by a mixture of a thermoplastic polyurethane elastomer and a thermoplastic polyamide elastomer. This technique involves difficulties in sufficiently increasing the abrasion resistance of an obtained golf ball due to poor miscibility between the thermoplastic polyurethane elastomer and the thermoplastic polyamide elastomer.